The invention relates to stabilizing the viscosity of alkaline solutions.
Polymeric viscosity stabilizers such as cellulosics, alginates, and biogums, are used in compositions as thickening agents. However, such polymneric viscosity stabilizers generally do not have long-term stability in highly alkaline aqueous systems such as in household cleaners. Such cleaners often contain from about 0.5 to about 15 wt % caustic materials, such as sodium hydroxide, and have a pH of at least 10, typically about 12 to about 14. In order to have a suitable shelf life, such cleaners require long-term viscosity stability, e.g. for more than 12 months.
If the solution is unstable, either gelation or complete loss of viscosity occurs. Currently, xanthan gum is often used in these systems because it has relatively good stability under alkaline conditions. However, xanthan gum can exhibit gelation at higher levels of caustic and will show significant degradation over time, especially at elevated temperatures. Therefore, it is desired to have a thickener that provides even higher stability than xanthan gum.
The present invention is directed to an aqueous alkaline solution comprising at least a caustic and a viscosity-stabilizing amount of diutan gum. Typical caustics used in cleaning solutions include sodium hydroxide, potassium hydroxide, ammonium hydroxide, sodium carbonate, or sodium silicate. Typically, the aqueous alkaline composition has a caustic concentration from about 0.5 wt % to about 15 wt %, based on the total weight of the solution, to achieve a pH of at least 10. The amount of the diutan gum is preferably from about 0.05 wt % to about 2 wt %, based on the total weight of the solution.
In further embodiments, the aqueous alkaline composition contains at least one of a sequesterant, surfactant, or organic solvent.
Another embodiment of the invention is directed to a method of increasing the stability of an aqueous alkaline composition comprising adding to the aqueous alkaline composition a viscosity stabilizing amount of diutan gum.
Diutan gum is heteropolysaccharide S-657, which is prepared by fermentation of a strain of Sphingomonas sp. ATCC 53159. It has thickening, suspending, and stabilizing properties in aqueous solutions. Heteropolysaccharide S-657 is composed principally of carbohydrate, about 12% protein and about 7% (calculated as O-acetyl) acyl groups, the carbohydrate portion containing about 19% glucuronic acid, and the neutral sugars rhamnose and glucose in the approximate molar ratio of 3:2. Details of the diutan gum structure may be found in an article by Diltz et al., xe2x80x9cLocation of O-acetyl groups in S-657 using the reductive-cleavage methodxe2x80x9d Carbohydrate Research 331 (2001) 265-270, which is hereby incorporated by reference in its entirety. Details of preparing diutan gum may be found in U.S. Pat. No. 5,175,278, which is hereby incorporated by reference in its entirety.
It was discovered that aqueous alkaline solutions containing diutan gum have superior stability when compared to aqueous alkaline solutions containing xanthan gum at different concentrations of the gums. It was further discovered that diutan gum can be used as a thickening agent in aqueous alkaline solutions used to prepare household and industrial cleaners.
Aqueous alkaline solutions useful for household and industrial cleaners have a pH of at least 10, typically from about 12 to about 14. The aqueous alkaline solutions typically contain at least one caustic agent such as sodium hydroxide, potassium hydroxide, sodium silicate, ammonium hydroxide, sodium carbonate, or mixtures thereof. Generally about 0.5 wt % to about 15 wt % of the caustic is present in the alkaline solution, preferably about 5 wt % to about 12 wt %, more preferably about 10 wt %, based on the total weight of the solution.
Diutan gum is added in an amount effective to thicken and stabilize the aqueous alkaline solution. Effective amounts include from about 0.05 wt % to about 2 wt %, preferably about 0.1 wt % to about 1 wt %, more preferably about 0.2 wt % to about 0.8 wt %, based on total weight of the solution.
The aqueous alkaline solutions further contain other ingredients useful in cleaners. Such solutions may contain effective amounts of surfactants used as foaming wetting agents, and detergents. Organic solvents such as glycols and glycol ethers such as polyethylene glycol (PEG) or butyl cellusolve may also be present to aid in the removal of organic deposits, stains or coatings. Sequesterants are also added as builders to boost the effect of the detergent and to solubilize polyvalents salts. Sequesterants such as sodium glucoheptonate are also helpful at controlling polyvalents like iron that, if unsequestered, may catalyze base hydrolysis of the biogum thickener and lead to viscosity loss. Sequesterants also help to prevent polyvalent induced gelation.